Electroluminescent displays are used as light emitting type electronic display device (ELD). As the constituting element of the ELD, an inorganic electroluminescent element (inorganic EL element) and an organic electroluminescent device (organic EL device) are cited. The inorganic EL device has been used as a planar light source though high alternative voltage is required for driving such the light emitting device.
The organic EL device is an element having a light emission layer placed between a cathode and an anode, in which electrons and positive holes are injected into the light emission layer and excitons are generated by recombination of them, and light (fluorescence or phosphorescence) is emitted on the occasion of quenching of the excitons. Such the device is noted because which can emit light by application of a voltage of several to several tens volts, and has wide viewing angle and high visibility since it is a self light emission type, and is completely solid state thin device suitable for space saving and portable appliance.
It is important character of the organic EL device that the element is a planar light source different from ordinary practically used principal light sources such as light emission diodes and cold cathode ray tubes. As the use for effectively utilizing such the character, illuminating light source and backlight for various displays are cited. Particularly, such the element is suitably used for the backlight of liquid crystal full color display which is strongly demanded in recent years.
When the element is used as the backlight of the full color display, the element is used as a source of white light. The following methods are applicable for obtaining white light by the organic EL device; a method in which plural light emission materials are used in an element so as to emit white light by color mixing, a method in which plural colors pixels such as pixels of blue, green and red are separately coated and simultaneously lighted for obtaining white light, and a method in which a color conversion dye such a combination of a blue light emission material and a color conversion fluorescent dye is used for obtaining white light.
The method in which the color emission materials are prepared as one element for obtaining white light is advantageous for the use of backlight from the viewpoint of the requirement for the backlight such as low cost, high production efficiency and simple driving method.
Such the method includes in detail a method using complementary two color materials such as a blue light emission material and a yellow color emission material in one device to obtain by color mixing and that using three color materials to obtain white light by color mixing. However, the method using three colors, blue, green and red, light emission materials is preferable from the viewpoint of color reproducibility and reducing light loss by color filter.
It has been reported that a white light emission organic EL element can be obtained by doping the fluorescent materials reach emits blue, green or red light with high efficiency as the light emission material; cf. Patent Publications 1 and 2 for example.
Moreover, development of phosphorescent light emission material is made progress since an element showing high luminance light emission can be obtained; cf. Non-patent Publications 1 to 3 for example. Such the effect causes that the light emission efficiency of the phosphorescent light emission material can be raised for 4 times in maximum of that of the fluorescent light emission material because the upper limit of internal quantum efficiency becomes 100% by the later mentioned formation ratio of the singlet to triplet exciton and the internal conversion from the singlet exciton to the triplet exciton compared with the case of the light emission in usual fluorescent material in which the light is generated from singlet excited state and the formation probability of the light emission exciton species is 25% since the formation ratio of the singlet exciton to triplet exciton is 1:3.
The organic EL element is markedly deteriorated in its light emission property by exposure to air or moisture. The organic EL element employing phosphorescence light mission material is also deteriorated markedly by exposure to air or moisture.
An organic EL panel is proposed which is formed by a method in which an organic layer is sealed with a sealing cap can such as a metal can by sticking it to the organic EL substrate with adhesive for the purpose of preventing to exposure to air or moisture to attain high durability and long life. This to provide sealing space in which inactive gas is filled and moisture absorption agent is also provided. (Reference is made a sectional view of illumination device employing conventional organic EL element of FIG. 7). Consequently though the organic EL element as itself is a very thin element, the device is thick as a whole to have thickness of the space to provide the sealing parts and moisture absorption agent.
An organic EL panel is proposed which employs sealing film having barrier characteristics capable to form a thin EL element on the other hand (for example, Patent Document 5).
Patent Document 5 proposes to seal by moisture proof film. Volume within sealed space changes by expansion and shrink due to temperature or atmospheric pressure since the sealed space is atmosphere pressure, and, adhesion part is suffered from stress which causes cracks or peeling at adhesion part by repeating the expansion and shrink, and therefore, oxygen or moisture penetrated from the outside, consequently the element is deteriorated. Further, pressure change by expansion and shrink influences to a barrier film to induce generation of cracks of the barrier layer.
This invention is performed to obtain a thin and compact organic EL device having high durability, particularly one using phosphorescent light emission material
Patent Document 1: JP A H06-207170
Patent Document 2: JP A 2004-235168
Patent Document 3: U.S. Pat. No. 6,097,147
Patent Document 4: WO 02/5,011,013
Patent Document 2: JP A H11-162634
Non-Patent Document 1: M. A. Baldo et al., Nature, 395, 151-154 (1998)
Non-Patent Document 2: M. A. Baldo et al., Nature, 403, 750-753 (2000)